1. Field of the Invention
The present invention relates to an apparatus and method for color conversion which are used in an image output device capable of outputting a color image corresponding to an input color image with a high color accuracy.
2. Description of the Related Art
In recent years, with spread of network environments in offices, digital copy machines have changed to multifunction peripherals (MFP) which are connected to network and which are usable not only as the copy machines but also as printers, scanners, and FAX. Especially, a specific gravity has increased in an application as the printer. Accordingly, in full-color digital multifunction peripherals (hereinafter referred to as the color MFP), not only a viewpoint for copy indicating that colors of drafts are faithfully reproduced but also performance of printer color reproduction indicating that the colors displayed in displays such as a monitor device are favorably outputted are regarded as important.
In general, as compared with a color reproduction range that can be displayed in the monitor device, a printer color reproduction range that can be outputted by the color MFP is small, and therefore colors outside a printer color range cannot be matched. Therefore, with respect to the monitor display colors outside the printer color range, a method of mapping the colors in appropriate colors in the printer color range has broadly been used.
In this case, a policy of the color reproduction is considered to be appropriate as “there is not any sense of incongruity from monitor display, and the reproduced color is clear and easy to see even when seen alone”.
During the mapping, to transform the color by a table referring system, a method is used in which image data itself of a color conversion object is used as a read address of a color conversion table to read a color conversion result or a reference value for interpolating the color conversion result from the color conversion table.
Moreover, for a usual flow in using the color MFP to print/output an RGB image file, RGB is transformed into L*a*b* or CIEXYZ (called a profile connection space (PCS)) which is a device independent color space, and further transformed into CMYK. This is two stages of conversion. In the color conversion, each ICC profile is used as information indicating identity of RGB and CMYK.
For example, an example in which an RGB image inputted from a scanner is transformed into a CMY image for a printer output will be described with reference to FIG. 16.
In FIG. 16, a painted-over portion in the color conversion-table indicates a region surrounded so as to include all colors appearing in a certain image, and a non-painted-over portion indicates a region which does not appear in the image, that is, which is out of the object of the color conversion.
The RGB image inputted from the scanner includes the color which appears and the color which does not appear in accordance with a scene content. Therefore, in many cases, even when the color region is surrounded so as to include all the colors appearing in a certain image (scene), all RGB spaces that are color spaces before the conversion are not included. In many cases, a region outside the color conversion object is rather large, although the region is in the RGB space.
For the accuracy of the color conversion by the table referring system, it is more satisfactory to use a table in which table entries (the number of lattice points inputted in the table) are denser, that is, a large number of table lattice points exist at short intervals.
It is to be noted that in “Image-Dependent Gamut Compression and Extension, Hiroaki Kotera et al., pages 288 to 292, PIC2002 proceeding (Imaging Science and Technology)”, a color conversion method has already been proposed in which gamut mapping is used to extend or compress a range of colors possessed by the input image, and the range is fully matched with a color range that can be outputted by an output device.
As described above, when the valid number of lattice points is limited only inside the region painted over in FIG. 16, the lattice points in the region that is not painted over are wasted. That is, as shown in FIG. 16, the number of table entries for use in the color conversion (this will be referred to as the valid lattice point number) is limited only inside the painted-over region. Moreover, the region painted over in FIG. 16 corresponds to a table in which the valid lattice point number is substantially coarse. Therefore, there is a problem that a memory for use in the table is wasted, whereas the color conversion accuracy drops.
On the other hand, in a color conversion flow in which the ICC profile is used, the following three problems are caused.
For example, in a stage in which a PCS value is transformed into a CMYK value, information indicating the PCS values corresponding to primary colors RGB is lost. Therefore, a process limited to the primary colors RGB becomes difficult.
Moreover, even if the PCS values corresponding to the primary colors RGB are known, in general, the PCS values are not put on a straight line connecting the lattice points of the color conversion table in a CMYK profile. Therefore, the CMYK value corresponding to the PCS value has to be obtained by table interpolation with the lattice points of another hue. There is a problem that the hue is distorted by an interpolation error.
Furthermore, when several types of RGB color spaces have to be handled as the color spaces of the input image, the PCS value corresponding to the primary color differs with each type of the RGB color space. Therefore, there is a problem that a plurality of CMYK profiles have to be prepared.